Methods in food analysis rui m s cruz, igor khmelinskii, margarida c vieira, CRC press, 2014 scan

Shear-thickening fl uids, also known as dilatant materials, are characterized by an apparent viscosity that increases with shear rate.. Thixotropic fl uids are characterized by an appar

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Methods in Food Analysis

Methods in Food Analysis

Taylor & Francis Group

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© 2014 by Taylor & Francis Group, LLC

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Version Date: 20140506

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Measurements of food quality parameters, such as physical, chemical, microbiological and sensory parameters are necessary to characterize both existing and newly developed food products, to avoid possible adulterations/contaminations, and thus, control their quality at every stage

of production/distribution or storage at industrial and laboratory scales Several methodologies are reported in literature that allow quantifying different quality parameters This book comprehensively reviews methods

of analysis and detection in the area of food science and technology It covers topics such as lipids, color, texture and rheological properties in different food products The book focuses on the most common methods

of analysis, presenting methodologies with specifi c work conditions The book is divided into seven chapters, each dealing with the determination/quantifi cation analyses of quality parameters in food products

It is an ideal reference source for university students, food engineers and researchers from R&D laboratories working in the area of food science and technology This book is also recommended for students at undergraduate and postgraduate levels in food science and technology

The editors would like to express their sincere gratitude to all contributors

of this book, for their effort to complete this valuable venture

Rui M.S Cruz Igor Khmelinskii Margarida C Vieira

1 Textural and Rheological Properties of Fruit and Vegetables 1

R.K Vishwakarma, Rupesh S Chavan, U.S Shivhare and Santanu Basu

1.7.2 Plate and Plate Viscometers 31

Jin-Yeon Jeong, Gap-Don Kim, Han-Sul Yang and Seon-Tea Joo

Lipid Analysis

3.6 Mass Spectrometric Based Methods for Vegetable Oil Analysis 69

4 Texture in Meat and Fish Products 76

Purifi cación García-Segovia, Mª Jesús Pagán Moreno

and Javier Martínez-Monzó

Sara M Oliveira, Cristina L.M Silva and Teresa R.S Brandão

Rui Pedrosa, Carla Tecelão and Maria M Gil

6.3 Analysis of Lipid Extracts from Fish and Meat Samples 168

6.3.2 Instrumental Methods for Lipid Characterization 185

7 Vibrational and Electronic Spectroscopy and Chemometrics in 201 Analysis of Edible Oils

Ewa Sikorska, Igor Khmelinskii and Marek Sikorski

7.3.3 Multivariate Quantitative and Qualitative Models 2147.4 Application of Spectroscopy and Chemometrics in the 222Analysis of Edible Oils

Textural and Rheological Properties of Fruit and

Vegetables

U.S Shivhare3 and Santanu Basu3,*

ABSTRACT

Texture and rheology properties, such as viscosity, are key properties which consumers evaluate while determining the quality and acceptability of fruit and vegetables and manufactured products Understanding food texture requires an integration of the physical, physiological and psychophysical elements of oral processing The knowledge of texture and rheology has many applications in the food industry, i.e., from designing aspects of equipment, bulk handling systems, to new product development and quality control of foods Texture can be defi ned as “the group of physical characteristics that arise from the structural elements of the food, and are sensed primarily by the feeling of touch, are related to the deformation, disintegration and fl ow of the food under a force, and are measured objectively by functions of mass, time, and distance” The rheological behavior of fl uid foods is determined by measurements of shear stress versus shear rate/time or elastic/viscous modulus versus frequency,

1 Central Institute of Postharvest Technology, P.O PAU, Ludhiana, Punjab, India.

2 National Institute of Food Technology, Entrepreneurship and Management, Plot No 97, Sector 56, HSIIDC Industrial Estate, Kundli, Haryana, India.

3 Dr SS Bhatnagar University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh 160014, India.

* Corresponding author

and representation of the experimental data by viscometric/oscillatory diagrams and empirical equations, as a function of temperature and/

or concentration The textural and rheological properties of fruit and vegetables are extremely important for plant physiologists, horticulturists, food scientists and agricultural/food engineers due to different reasons

a number of rheological and other properties of foods and their interactions

(McCarthy 1987) According to Bourne, the textural properties of a food

are the “group of physical characteristics that arise from the structural elements of the food that are sensed by the feeling of touch, are related to the deformation, disintegration, and fl ow of the food under a force, and are measured objectively by functions of mass, time, and distance” (Bourne 1982) The terms texture, rheology, consistency, and viscosity are often used interchangeably, despite the fact that they describe properties that are somewhat different In practice the term texture is used primarily with reference to solid or semi-solid foods rather than liquids

Rheology is the science of deformation and fl ow of matter It is the study

of the manner in which a fl uid responds to applied stress or strain (Steffe 1996) Rheology may be defi ned as the study of deformation and fl ow of matter or the response of materials to stress (Bourne 1993) Rheology of a product is related to the fl ow of fl uids and the deformation of matter The science of rheology has many applications in design of food processing equipment and handling systems such as pumps, piping, heat exchangers, evaporators, sterilizers, and mixers, as well as in product development and quality control of foods (Saravacos 1970; Rao 1977; 1987) A number

of food processing operations depend greatly upon rheological properties

of the product at an intermediate stage of manufacture because this has a profound effect upon the quality of the fi nished product The microstructure

of a product can also be correlated with its rheological behaviour allowing development of newer products In particular, food rheologists have made unique contributions to the study of mouth-feel and its relation to basic rheological parameters (Rao 1986)

Determination and evaluation of the textural properties of solid foods present many diffi culties to scientist, as described by Prins and Bloksma (1983) Agricultural materials are generally inhomogeneous, anisotropic and inelastic in nature Therefore, their behaviour under loads may vary and stresses may affect other parts of material rather than affecting homogeneous materials such as metals Since agricultural materials and food products deform in response to applied forces, the nature of the response varies widely among different materials It depends upon many factors including the rate at which force is applied, the previous history

of loading, the moisture content and the composition The force required

to produce a given amount of deformation may be used to quantitatively evaluate the texture of raw and processed foods In case of raw produce, it may be used for developing new varieties, as a criterion for determining those varieties that have desirable texture Force-deformation testing is used

to study damage which occurs during harvesting, handling, and processing Such studies often give insight into the specifi c circumstances that lead

to failure and how such failure may be prevented Sample history can be crucial, with results dependent not only on factors such as rate and extent

of deformation but also on the sample history, which includes processing and storage effects prior to measurement

Knowledge of mechanical properties is useful for both manufacturers and users of food-processing equipment Knowing the ultimate resistance of

a product to mechanical loads helps in saving the products from mechanical damage such as bruising (Brusewitz et al 1991) On the other hand, food processors need to apply required loads for work to be done For example, applying the minimum force to cut the peel at the peeling stage of food processing is a matter of importance Knowing the minimum load helps producers save energy and optimise equipment design Therefore adequate knowledge of the textural and rheological properties of fruit and vegetable (or their transformation products) is important for storage conditions, process equipment design and quality control

1.2 Concepts of Stress and Strain

Food materials will deform or fl ow on application of stress Stress (σ) is defi ned as the force (F, N) divided by the area (A, m2) over which the force

is applied, and is generally expressed as Pa Direction of the force with respect to the surface area impacted determines the type of stress If the force

is directly perpendicular to the surface, a normal stress develops tension

or compression in the material If the force acts in parallel to the sample surface, shear stress is experienced

Strain is a dimensionless quantity representing the relative deformation

of a material The direction of the applied stress with respect to the material

surface determines the type of strain If the stress is normal (perpendicular)

to a sample surface, the material will experience normal strain (ε) (Steffe

1996; Daubert and Foegeding 1998) Foods show normal strains when they

are compressed (compressive stress) or stretched (tensile stress) Normal

strain (ε) may be calculated as a true strain by integration over the deformed

length of the material

The principles of rheology are commonly applied to understand and

improve the fl ow behavior and textural attributes of food materials and to

reveal relationships between the physical properties and the functionality

of the material (Steffe 1996) Rheology attempts to build relationships

between forces and corresponding deformations, and is expressed more

fundamentally as shear stress and shear strain

1.3.1 Shear Stress

Shear stress (τ) is defi ned as a force (F, N) per unit area (A, m2) Stress is

commonly given in Pascal (N/m2), and expressed as

Shear strain occurs when stress is applied parallel to the material surface

Shear strain (γ) is the inverse tangent of the change in distance (Δd) divided

by the initial height (h) of the material.

For fl uids, a shear stress can induce a unique type of fl ow called shear

fl ow The differential change of strain (γ ) with respect to time (t) is known

as shear (strain) rate (γ , s–1)

γ

dt

dγ

1.3.4 Viscosity and Apparent Viscosity

Viscosity, also called dynamic viscosity or absolute viscosity, of a fl uid is

essentially its internal friction to fl ow, and rheology provides information

about the internal molecular structure of a system Viscosity (η, Pa.s) is

Rheological behavior of fl uids is characterized by measurement of

viscosity If a plot of shear stress (τ) vs shear rate (γ ) results in a straight line,

viscosity (η) is constant and that material is classifi ed as Newtonian (Fig 1.1)

Fluid that does not obey this relationship (τ = η.γ ), is non-Newtonian, which

includes most of the food materials According to a standard classifi cation

of non-Newtonian fl uids or fl ow behavior, there are three main classes: time

independent (steady state), time dependent, and viscoelastic fl uids, where

the fl ow is viscoelastic (Fig 1.1)

Apparent viscosity (η a, Pa.s) is the measure of resistance to fl ow or the

fl uidity of a non-Newtonian fl uid and is the ratio of shear stress to shear

rate It is a coeffi cient calculated using eq (1.5) from empirical data as if

the fl uid obeyed Newton’s law Most materials exhibit a combination of

two or more types of non-Newtonian behaviour (Lapasin and Pricl 1995;

Steffe 1996)

Figure 1.1 Time-independent fl ow behavior.

Dilatant Newtonian Pseudoplastic

Dilatantwith yield point

Pseudoplasticwith yield point

Bingham Pseudoplastic with yield point

Dilatant with yield point

Time-independent fl uids are materials with fl ow properties that are independent of the duration of shearing These fl uids are further subdivided into three distinct types:

Shear-thinning or pseudoplastic fl uids are characterized by an apparent

viscosity which decreases with the increasing shear rate, while the curve begins at the origin (Fig 1.1) The rate of decrease in viscosity is material-specifi c

Shear-thickening fl uids, also known as dilatant materials, are characterized

by an apparent viscosity that increases with shear rate This trend is fairly rare in foods

Viscoplastics fl uids are those that exhibit yield stress (τ0), which is a unique feature of plastic behaviour Yield stress is a limiting shear stress at which the material begins to fl ow, while below this yield value the material behaves as an elastic solid

Time-dependent fl uids are materials in which the shear fl ow properties

depend on both the rate and the time of shearing There are many food products that recover the original apparent viscosity after a suffi cient period

of rest, while in others the change is irreversible This type of fl uid behaviour may be further divided in two categories: thixotropic and rheopectic

Thixotropic fl uids are characterized by an apparent viscosity that decreases

with time when sheared at a constant shear rate The change in apparent viscosity is reversible, that is, the fl uid will revert to its original state on rest During shearing, the apparent viscosity of the system decreases with time until a constant value is reached and this value typically corresponds

to the point where there is no further breakdown of structure Examples: jam, jelly, marmalade, fruit pulp/juice, cheese etc

Rheopectic (or anti-thixotropic) fl uids are materials in which the apparent

viscosity of fl uid increases with time when subjected to a constant shear rate This phenomenon is often an indication of aggregation or gelation that may result from increasing the frequency of collisions or a more favourable position of particles Examples are rare in food, one is such as starch solution under heating

Viscoelastic fl uids are materials that are simultaneously viscous and

elastic Most food materials exhibit some viscous and some elastic behaviour simultaneously and are therefore referred to as viscoelastic (Gunasekaran and Ak 2000) The viscoelastic properties of materials may be determined using dynamic or transient methods The dynamic methods include frequency sweep and stress/strain sweep The transient methods include stress relaxation (application of constant and instantaneous strain and measuring decaying stress with respect to time) and creep (application of constant and instantaneous stress and measuring increasing strain with time)

1.3.5 Shear Modulus

Shear modulus (G, Pa) is the constant of proportionality used to relate shear

stress with shear strain (Steffe 1996)

a combination of these stresses, such as fatigue (Fig 1.2)

Compressive stresses develop within a material when forces compress

or crush the material When a food material is placed between two plates and plates are moved towards each other, the food material is under compression

Tension (or tensile) stresses develop when a material is subject to a pulling load; for example, using a wire rope to lift a load or when anchoring

an antenna Tensile strength is defi ned as resistance to longitudinal stress

Figure 1.2 Stress applied to a material.

Compression Tension Shear Torsion Bending (impact) Fatigue

For example, a noodle can be broken by hand by bending it back and forth several times in the same place; however, if the same force is applied in a steady motion (not bent back and forth), the noodle cannot be broken The tendency of a material to fail after repeated bending at the same point is known as fatigue.

1.4.1 Stress-Strain Relationship

Rheologically, the question as to whether a particular food is a solid or a liquid is considered in terms of the non-dimensional Deborah number (D), defi ned by Reiner (1964) as the ratio of the relaxation time of the sample divided by the time of observation The difference between solids and fl uids

is then described by the magnitude of D Time of observation is, in general,

a crucial variable when investigating the mechanical properties of foods If the time of observation is very long or, conversely, if the time of relaxation

of the material under observation is very small, the material will fl ow and will be liquid On the other hand, if the time of relaxation of the material is larger than time of observation, the material, for all practical purposes, is a solid It is thus necessary to determine not just a stress-strain curve but the stress-strain-time relationships describing the behaviour of the material For complex foods there is an artifi cial distinction between solid and liquid states, which depends not only on the material but also on the experimental time scale relevant to the specifi c use of the food or the specifi c process to which the food is subjected

When force is applied to a solid material and the resulting stress versus strain curve is a straight line through the origin, the material is obeying Hook’s law The relationship may be stated for compressive stress and strain as

The constant E is also known as Young’s modulus of elasticity and

describes the capability of a material to withstand load Hookean materials

do not fl ow and are linearly elastic Strain remains constant until stress is removed and material returns to its original shape However, most of the food materials follow the Hook’s law for small strains only, typically below 0.01 Large strains often produce brittle fracture or non-linear behaviour

In addition to Young’s modulus of elasticity, Poisson’s ratio (ν) is

determined from the compression tests

ν = Lateral Axial strain strain eq (1.8)

Poisson’s ratio may range from 0 to 0.5 Typically ν varies from 0.0 for

rigid-like materials containing large amounts of air to near 0.5 for liquid-like materials Values from 0.2 to 0.5 are common for biological materials with 0.5 representing an incompressible substance like potato fl esh

1.4.2 Compression Test of Food Materials

Uniaxial compression is a popular method of testing agricultural materials because the shape of the specimen simplifi es the calculation of normal stresses and modulus of elasticity Since food materials are non-homogeneous, the term apparent modulus of elasticity is used in place of modulus of elasticity Mohsenin (1986) observed that under small strains, most agricultural materials exhibit extensive elasticity, to which Hertz’s theory of contact stress is applicable The original analysis of elastic contact stresses, by H Hertz, was published in 1881 and later translated into English by Jones and Schott (Hertz 1896) Defl ection occurs when a collinear pair of forces presses the two elliptical bodies together and the point of contact is replaced by a small elliptical area of contact (Hertz 1896) (Fig 1.3) The equations simplify when the contact area is circular such as

Figure 1.3 Hertz problem (above) for two convex bodies in contact, (below left) sphere on a

fl at plate, (below right) sphere on sphere (Adapted from Mohsenin 1986).

with two spheres or sphere and plate whose principal plane of curvature coincide To solve the problem, the size and shape of contact area as well

as the distribution of normal pressure acting on the area are determined The defl ections and subsurface stresses resulting from the contact pressure are then evaluated with certain fundamental assumptions made to solve the problem (Hertz 1896; Mohsenin 1986) These assumptions are: (i) the material is homogeneous; (ii) contact stress is over a small area relative

to the material size; (iii) radii of curvature of the contacting surfaces are substantially greater than radius of the contact area; and (iv) the surfaces are smooth

Determination of compressive properties requires the production of

a complete force-deformation curve From the force-deformation curve, stiffness, apparent modulus of elasticity, toughness, force and deformation

to points of infl ection, to bio-yield, and to rupture, work to point of infl ection, to bio-yield, and to rupture, and the maximum normal contact stress at low levels of deformation may be obtained Any number of these mechanical properties can be chosen for the purpose of evaluation and quality control

When a food material is subjected to compression, it may rupture after following a straight force-deformation curve (Fig 1.4) (ASAE 1998) The point at which rupture takes place is known as bio-yield point It is the point where an increase in deformation results in a decrease or no change in force (Fig 1.4) In a brittle material, rupture may occur in the early portion

of the force-deformation curve beyond the linear limit, while it may take place after considerable plastic fl ow in a tough material (Mohsenin 1986)

Figure 1.4 Force-deformation curves for materials with and without bioyield point PI=point

of infl ection, D =deformation at point of infl ection (Adapted from ASAE 1998).

Toughness is defi ned as the ability of a material to absorb energy before

fracture This can be approximated by the area under the stress-strain or

force-deformation curve up to the point of rupture (Mohsenin 1986)

Apparent Modulus of Elasticity

The apparent modulus of elasticity of the bodies of convex shape may be

determined using Hertz equations (Seely and Smith 1965; Timoshenko and

Goodier 1970; Mohsenin 1986) It is always determined before the point

of infl ection The point of infl ection is the point at which rate of change of

slope of the curve becomes zero (Fig 1.4) The combined deformation (δ)

of the two bodies along the axis of load is expressed as:

1 3 2

where R1 is the maximum radius of curvature of the body 1 (mm); R'1 is the

minimum radius of curvature of body 1 (mm); R2 is the maximum radius of

curvature of body 2 (mm); R'2 is the minimum radius of curvature of body 2

(mm); k is a factor depending on the curvature of bodies (dimensionless); F

is force applied (N); δ is combined deformation of both bodies (mm); and,

E c is apparent modulus of elasticity (MPa)

The E c is the contact modulus and is expressed as:

where ν1 and ν2 are Poisson’s ratios of bodies 1 and 2; and E1 and E2 are

apparent moduli of elasticity of body 1 and 2 respectively

The major and minor axes of the elliptical contact area can be calculated

using equations (1.11) and (1.12)

1 3 1

The values of k, m, and n depend on the principal curvatures of the

bodies at the point of contact and the angle between the normal planes

containing the principal curvatures The values of k, m, and n are available

in literature for various values of angle (θ) between the normal planes

containing the curvatures (Timoshenko and Goodier 1970; Mohsenin 1986;

ASAE 1998)

The maximum contact stress occurs at the centre of the surface of contact

(the fi rst point of contact between the compression tool and the sample) It

is numerically equal to 1.5 times the average contact pressure and can be

calculated from following equation

In case of nearly spherical food materials (approximated as a sphere

compressed between two large rigid plates, with the principal planes of

curvature coinciding), the following is valid:

(i) for a spherical body (plant seed) with diameter D g : R 1 =R 1 ’=R=D g /2;

For the special case of a rigid plate of metal, E 2 (of compression tool)

is much higher than E 1 (of material) The contact modulus can therefore

Using these values in eq (1.7) and rearranging, the apparent modulus

of elasticity of the material is expressed as:

Bio-yield Point for Spherical Materials

The Hertz equations are used to predict the failure of food materials under

quasi-static compressive loading At bio-yield point, radius of contact circle

(α) is computed using equation (1.16) (Timoshenko and Goodier 1970;

Shigley and Mishke 2001)

The maximum stress occurs on the axis of loading at the centre of the

contact area where the two bodies fi rst come into contact It is numerically

equal to 1.5 times the mean stress and is given by equation (1.17) (Seely and

Smith 1965; Timoshenko and Goodier 1970; Shigley and Mishke 2001)

The material tends to expand in the x- and y- directions when compressed

normal to the axis of compression (z-direction) The surrounding material,

however, does not permit the expansion, and compressive stresses are

produced in x- and y-directions The two planes of symmetry in loading

and the spherical geometry dictate that principal stresses σ x =σ y and σ z =σmax

occur at the point of contact The principal stresses at a distance z below the

surface along the compression axis are given by the following expressions

(Timoshenko and Goodier 1970; Shigley and Mishke 2001)

Therefore, the maximum shear stresses developed are represented by

equations (1.20) and (1.21) (Seely and Smith 1965; Timoshenko and Goodier

1970; Shigley and Mishke 2001)

The maximum shear stress is developed on the load axis, approximately

0.48α below the surface Ductile materials fi rst yield at the point of maximum

shear stress (Timoshenko and Goodier 1970; Shigley and Mishke 2001) The

values of stress components below the surface may be plotted as a function

of maximum stress of contacting spheres

The normal displacement (approach of distant points on the two bodies)

is given by following expression

Quasi-static compression tests may be performed using a universal

testing machine or a texture analyser In case of small-sized materials,

the material to be tested may be glued to the base plate (ASAE 1998) For

example, an individual seed is loaded between two parallel plates and

compressed until the seed fails (ASAE 1998; Saiedirad et al 2008) The slow speed of the compression tool allows the material to be compressed for an appreciable time before failure occurs The point of infl ection may be determined visually from the force-deformation curve (Fig 1.4) to compute apparent modulus of elasticity (Mohsenin 1986; ASAE 1998; Sayyah and Minaei 2004) For conducting compression tests, the procedures prescribed

by ASAE (ASAE 1998) should be followed

Factors Affecting Force Deformation Behaviour

Moisture content of the material plays a signifi cant role in mechanical properties of food materials Moisture would also greatly affect the stress-strain behaviour of dried food products such as spaghetti noodles or crackers Such materials typically have moisture ranging from 5 to 30%, while fruit and vegetables have moisture contents of 75–90% (wet basis) Strain rate also affects the stress-strain behaviour of agricultural materials and food products More stress is usually required to produce

a given amount of strain at higher strain rates This is true for grains and seeds as well as dry food material The behaviour of fruit and vegetable tissue is more complex When the cells are ruptured, more stress is required

to produce a given amount of strain at the faster loading rate Strain rate has a relatively small effect at the intermediate water potential

Compression of agricultural materials and food products usually produces a relatively large plastic strain As a result, their stress-strain bahaviour changes under repeated or cyclic loading Most of the plastic strain occurs during the fi rst cycle of loading

1.4.3 Stress Relaxation

If agricultural materials and food products are deformed to a fi xed strain and the strain is held constant, the stress required to maintain the deformation

decreases with time This is called stress relaxation For example, in the

behaviour of a cylindrical sample of potato tissue at a strain of 10% the decrease in stress is extremely rapid during the fi rst 5 or 10 seconds of loading The initial stress is approximately 0.6 MPa, which decreases to 0.1 Mpa in just 2 seconds (Pitt 1984) The additional decrease during the next 18 minutes is relatively small The stress as a function of time may be described by a sum of a constant and one or more exponential terms

1.4.4 Creep

In bulk handling situations, such as when potatoes are placed in a pile

or blocks of cheese are placed on the top of one another, a constant load

is applied to agricultural materials and food products If the stresses are relatively large, the material will continue to deform with time This increase in strain is called creep There is an almost instantaneous initial deformation followed by continual increases in strain as time of loading increases However, the rate of change in stress with time (the slope of the curve) decreases exponentially with time Eventually, the relationship between strain and time becomes nearly linear This type of behavior is also typical of fruit and vegetables

1.4.5 Deformation Testing Using Other Geometries

In many situations, agricultural materials are not loaded in simple compression Although compression at the surface imposes compressive stresses in the vicinity of the applied load, other portions may be under tension If fruit or vegetables absorb water rapidly, they may expand, producing tensile forces in the skin When grains and seeds are dried, the outer layers may shrink producing tensile stresses in the layers and in the surrounding pericarp For such agricultural materials and food products, the response under tensile loading may be very different from the response under compressive loading Furthermore, compression, shear, and tensile forces occur in many loading situations and when evaluating failure (breaking apart), it may be desirable to test the response of the material to all three types of forces

of tensile tests Tensile tests have mainly been performed for meat analysis where breaking strength is the best parameter for predicting tenderness in cooked meat

Tensile testing is less common than compression testing because it

is more diffi cult to grip the sample in such a manner that a tensile load can be applied If the ends of a bar of uniform cross-section are clamped, compression of the ends causes stress concentrations to develop, which promote failure in the vicinity of the clamp This problem is usually

overcome by making the sample wider in the vicinity of the clamp (Fig 1.5A) Ideally, the sample will fail at its narrowest point Therefore, stress is calculated from the minimum cross-sectional area In some samples, a punch

can be used to place a mark at two points on the sample and deformation (L)

becomes the change in distance between the two points A device called an

extensometer can also be used to measure L The extensometer is clamped to

the sample at two points and it automatically registers the change in length

on a dial gauge The engineering strain can be calculated from L.

Figure 1.5 Methods for tensile testing of agricultural materials and food products A Typical

sample shape used (The sample is wider at the ends where it is clamped into the testing device);

B Technique for tensile testing of noodles (Cummings 1981); C Technique for testing tomato skins (Murase and Merva 1977); D Apparatus used to conduct tensile tests on a single corn

kernel (Ekstrom et al 1966).

α

Sample clampedatends

Sample clampedatends

Pin

Loop for Pin

Loop Swivel

Brass Tube

Noodle Glue used

to fasten noodle in tube

Bottom of tension jig (anchored)

D

Sample clamped at ends

Sample clamped at ends

Aluminium

Severed foil

bottom half

Some of the most frequent applications of tensile testing are testing

of pericarp of grains and seeds or the epidermis of fruit and vegetables in which the sample is cut into the shape shown in Fig.1.5A (Liu et al 1989 for soybean seed coats) Cummings and Okos (1983) tested noodles using rapid-setting glue to secure them in a metal tube slightly larger in diameter than the noodles (Fig 1.5B) The tubes were attached to a tensile testing machine with wires and loops which allowed the sample to align itself with the line

of application of the force Murase and Merva (1977) wrapped tomato skin samples in aluminium foil, clamped the ends, severed the aluminium foil

to expose the sample, and then soaked the sample in solutions of known water potential while applying a tensile force (Fig 1.5C) Notches may also

be made in a corn kernel and tensile force applied with a special supporting jig (Fig 1.5D) (Ekstrom et al 1966)

1.4.7 Fracture Test

Fracturability is the parameter that was initially called “brittleness” It is the force with which a sample crumbles, cracks or shatters Foods exhibiting fracturability are products that possess a high degree of hardness and low degree of adhesiveness The degree of fracturability of a food is measured

as the horizontal force with which a food moves away from the point where the vertical force is applied Another factor that helps in determining fracturability is the suddenness with which the food breaks

1.4.8 Cutting and Shearing Test

There are many single-blade or multi-blade fi xtures available with universal testing machines or texture analysers that cut or shear through the food samples The maximum force required and the work done is taken as an index of fi rmness, toughness or fi brousness of the sample Although the term

“shear” is used to describe the action of such fi xtures, both compression and tension forces are developed as well Cutting and shearing test is usually done for foods with a fi brous structure, which includes meat, meat products and vegetables

1.4.9 Bending and Snapping Test

Bending is a combination of compression, tension and shear Snap test, defi ned as breaking suddenly upon the application of a force, is a desirable textural property in most crisp foods, such as fresh green beans and other vegetables, potato chips and other snack items The ability to snap is a measure of the temper of chocolate, the moisture content of crisp cookies,

the turgor of fresh vegetables and the amounts of shortening in baked goods The sharp cracking sound that usually accompanies snapping is the result

of high-energy sound waves generated when the stressed material fractures rapidly and the broken parts return to their former confi guration

Bending tests are performed by cutting a sample in the shape of a beam and placing it on a stand which supports it at two points separated

by a distance L (Fig 1.6) If a known force is applied to the centre of the

beam, the modulus of elasticity may be determined from the defl ection at

the point of application of force E may be calculated from the formula for defl ection of a simply supported beam having a cross-section of height h and width b Assuming that the force F is applied in the direction h so that the neutral axis of the beam is at h/2, and that the beam cross-section has the moment of inertia I, about the neutral axis, the value of E is given by

Figure 1.6 Three-point bending test on a sample cut into a rectangular cuboid The sample

is “simply supported” by the edges of the two supports and compressed by the edge of the compression tool attached to a movable cross-head Compression tool applies force midway

between the two supports

Sample of width b

Eq (1.23) is only valid when E in compression is equal to E in tension

Therefore, the bending test may not be appropriate for the determination

of E in many food materials

The bending test may be used to determine the critical tensile stress at

failure For a simply supported beam with a force F applied halfway between the supports, the maximum tensile stress, σmax, occurs at the bottom surface

of the beam (the surface opposite to that on which the force is applied) The

neutral axis is the plane where σ=0 If c is the distance from the surface to the neutral axis (c=h/2 for the simply supported beam) and M is moment

about the neutral axis, then

3 2

c

1.4.10 Puncture and Penetration Test

In a puncture or penetration test the probe penetrates into the test sample

by a combination of compression and shear forces that cause irreversible changes in the sample The puncture test measures the force required to reach a specifi ed depth, whereas penetration test measures the depth of penetration under a constant load In this test, the force necessary to achieve

a certain penetration depth is measured and used as a measure of hardness,

fi rmness or toughness Puncture and penetration tests are commonly used

in the testing of fresh fruit and vegetables, cheese, confectionery and the spreadability of butter and margarine Penetration tests, such as the Bloom test, have also been used extensively for testing the rigidity of gels

1.4.11 Texture Profi le Analysis (TPA)

Texture profi le analysis is also known as the two bite test A number of product characteristics may be quantifi ed and standard methods have been established to evaluate parameters such as adhesiveness, cohesiveness and springiness of food products This test is usually performed in compression

In this test the specimen is compressed to the point where material reaches the bioyield point (fi rst bite) and then the force is removed gradually After

a relaxation time, the force is again applied (second bite) till material fails Typical texture profi le analysis curve for pears is shown in Fig 1.7

The various parameters determined from the TPA tests are described below.

Hardness: It is the force necessary to attain deformation; given as the fi nal peak of the TPA curve (Fig 1.6), which is the force value corresponding to the

fi rst major peak (the maximum force during the fi rst cycle of compression)

It is also known as fi rmness

Fracturability: It is the force at which the material fractures (height of the fi rst signifi cant break in the peak of TPA curve) A sample with a high degree of hardness and low cohesiveness will fracture This is also called brittleness Fracturability is the force value corresponding to the fracture peak (if there is one)

Springness: It is the height that the food recovers during the time that elapses between the end of the fi rst cycle and the start of the second cycle The rate at which a deformed sample goes back to its undeformed condition after deforming force is important This is also called elasticity

Stringness: It is defi ned as the distance that the product is extended during de-compression before separating from the probe

Adhesiveness: It is the measure of the work necessary to overcome the attractive forces between the surface of the sample and surface of the probe

Figure 1.7 Typical texture profi le analysis curve.

Force

with which the sample comes into contact If adhesiveness is larger than cohesiveness, then part of the sample will adhere to the probe.

Cohessiveness: It is the measure of the strength of internal bonds making

up the body of the sample If adhesiveness is smaller than cohesiveness, then the probe will remain clean as the product has the ability to hold together

Chewiness: It is the measure of the energy required to masticate a solid sample to a steady-state of swallowing (Hardness × Cohesiveness × Adhesiveness)

semi-Gumminess: It is the measure of the energy required to disintegrate a solid sample to a steady state of swallowing (Hardness × Cohesiveness)

semi-1.4.12 Torsional Loading

The torsion test is a method well-suited for determining the failure properties of fruit, vegetables and other food products such as protein gels (Diehl et al 1979; Hamann 1983) It is particularly useful because the shear and normal stresses are equal during loading and therefore the plane of failure indicates which of these loadings is most likely to cause failure

In the torsion test, undesirable stress concentrations develop at the points of application of the torque at the ends of the sample Therefore, a sample of varying diameter is used The values of normal stresses/strains and shear stresses/strains may be calculated from the formulas summarized

by Hamann (1983) The choice of formula depends on the assumptions made

about the relationship between the applied moment, M and the angle of twist, Ψ If M is assumed to be a linear function of Ψ all the way to the point

of failure, then the values for the maximum shear stress, τmax and maximum

shear strain γmax, are given by:

1.4.13 Test Specimen and Testing Conditions

Determination of mechanical properties of food materials is a technical job

in which precision plays an important role The following points must be considered when testing food materials

• Specimens should be tested in their original size and shape.

• The specimen may be tested under 3 different conditions: (1) fresh, (2)

frozen and thawed, or (3) cooked and dried

• Tests on fresh specimens must be conducted before the time of exposure

to air exceeds 10 min in order to avoid changes caused by drying of

the specimen

• Frozen specimens must be thawed, brought to room temperature

(22 ± 2 ºC), and tested before drying occurs

• Cooked specimens should be air-dried for about 24 hours at room

temperature before testing

• Because of the large variance inherent in specimens, each experiment

must be statistically designed to have enough test specimens for

an acceptable level of confi dence in the results A minimum of 25

specimens should be used

• For shear tests, a crosshead speed of 5 mm/min should be used

• For the bending test, a crosshead speed of 10 mm/min should be

used

1.5 Steady State Rheology

Steady state relationship between shear stress-shear rate of food materials is

expressed in terms of the power law model or the Herschel-Bulkley model

The Herschel-Bulkley model is used for yield stress fl uids Yield stress

fl uids behave like a solid until a minimum stress, known as yield stress, is

overcome for beginning of the fl ow of the material (Fig 1.1)

where τ is shear stress (Pa), τo is yield stress (Pa), Iis shear rate (s–1), K is

consistency index (Pa.sⁿ) and n is fl ow behaviour index (dimensionless)

expressing the extent of deviation from Newtonian behaviour

Dependence of the fl ow behaviour of fl uid foods on temperature can

be described by the Arrhenius relationship (Saravacos 1970; Rao 1986;

where A K is frequency factor (Pa.sn), E K represents activation energy (J/mol),

R is gas law constant (R = 8.314 J/mol K), and T is absolute temperature

(K)

Yield stress is the point when the shear stress-shear rate curve starts

showing deviation of shear rate from zero This condition indicates initiation

of fl ow and the corresponding shear stress is taken as the yield stress

1.5.1 Time Dependent Rheology

Time dependent shear stress decay characteristics have been mathematically

described by several researchers (Weltman 1943; Hahn et al 1959; Figoni

and Shoemaker 1983; Nguyen et al 1998) The Weltman model (1943)

assumed logarithmic decay of shear stress in the absence of any equilibrium

condition The Weltman model was later modifi ed by Hahn et al (1959)

to include an equilibrium shear stress term Figoni and Shoemaker (1983)

described the stress decay process with a fi rst-order kinetic model with

a non-zero equilibrium value The structural kinetic model (Nguyen et

al 1998; Abu-Jdayil 2003) postulates that the change in the rheological

properties is associated with shear-induced breakdown of the internal

fl uid structure To quantify the time dependence of mango jam at selected

shear rates and temperatures, shear stress and time of shearing data were

fi tted to the Weltman, Hahn, Figoni and Shoemaker, and structural kinetic

where τ is shear stress (Pa) at any given time of shearing (t) The parameter

A represents the initial stress while B is time coefficient of structure

breakdown

Hahn Model

Hahn et al (1959) evaluated the Weltman model and found plots of τ versus

ln(t) for the mineral oil to be sigmoidal but not linear They argued on

theoretical basis that stress decay of thixotropic substances should instead

follow the fi rst-order type relationship

where τ e is the equilibrium shear stress value, which is reached after a long

shearing time; P represents the initial shear stress and a indicates the rate

of structural breakdown for the sample

Figoni and Shoemaker Model

Figoni and Shoemaker (1983) suggested a thixotropic model based on their

work on transient rheology of mayonnaise

max

where, τmax is initial shear stress; (τmax–τ e) represents the quantity of

breakdown structure for shearing; and k is a kinetic constant of structural

breakdown

Structural kinetic model

The observed time-dependent fl ow behaviour of the food materials is

also modelled using the structural kinetic approach (Nguyen et al 1998;

Abu-Jdayil 2003) This model postulates that the change in the rheological

properties is associated with shear-induced breakdown of the internal

fl uid structure in the food Using the analogy with chemical reactions,

the structural breakdown process may be expressed as breakdown from

structured to non-structured The rate of breakdown of the structure

during shear depends on the kinetics of the above reaction Based on the

experimental results from the transient measurements at constant shear

rates, and from the step change in shear rate measurements, it may be

assumed that the thixotropic structure in food breaks down irreversibly

without signifi cant build-up

Let Ψ = Ψ(γ, t) be a dimensionless parameter representing the structured

state at any time t and under an applied shear rate γ The rate of structural

breakdown may be expressed as

where k = k(γ) is rate constant; α is function of shear rate (γ); and m is the

order of the breakdown ‘reaction’ Initially, at the fully structured state, t =

0; Ψ = Ψ o ; and at steady state t = t: Ψ = Ψ α At a constant applied shear rate,

integration of equation (35) from initial time (t = 0) to a time (t) yields

To apply equation (1.35) to the experimental transient viscosity data, a

relationship between Ψ and measurable rheological quantities needs to be

determined Ψ may be defi ned in terms of the apparent viscosity (η) as:

where, η o is initial apparent viscosity at t = 0 (structured state); η is the

apparent viscosity at time t; and, η α is the fi nal or equilibrium apparent

viscosity at t→α (equilibrium structured state) Both η o and η α are functions

of the applied shear rate only

Substituting equation (1.36) into equation (1.35) gives the expression,

for a fi xed shear rate:

The form of equation (1.37) allows a simple way for testing the validity

of the model and determination of the model parameters m and k Equation

(1.37) is valid only under the constant shear rate condition (Nguyen et al

1998)

1.6 Viscoelasticity

Measurement of viscoelastic properties of food materials may be carried out

using dynamic oscillation of shear stress or strain (Steffe 1996; Gunasekaran

and Ak 2000) Harmonic oscillation of shear stress involves a material

being oscillated sinusoidally with varying stress while the resulting strain

is measured (Zhong 2003) Dynamic tests are carried out based on four

assumptions: (i) a constant stress or strain throughout the sample; (ii) no

slip of the sample; (iii) sample homogeneity; and (iv) measurements are

performed within the linear viscoelastic region Further, key parameters

like storage modulus, loss modulus, complex modulus, and phase angle

generated during dynamic oscillation studies describe the viscoelastic

behaviour of a material

Storage and Loss Modulus

The storage modulus (G') indicates the degree of elastic behaviour in a

material Shear storage modulus is the component in phase with the strain,

or the elastic behaviour

'

.cos

Loss modulus (G’) is the component out of phase with the strain or

viscous behaviour Thus shear loss modulus is an indication of the viscous

properties of the material

'' sin

Complex Modulus

The complex shear modulus (G*) includes both storage and loss moduli

values and is an indicator of the strength of a gel

G* = (G'2+G''2)0.5 eq (1.40)

Phase Angle

Phase angle (F) is directly related to the energy lost per cycle divided by the energy stored per cycle (Steffe 1996) Phase angles can vary from 0 to 90°, with 0° indicating an ideal solid material (Hookean solid) and 90° indicating

an ideal viscous material (Newtonian fl uid) Phase angle is expressed as:

in compression/tensile mode and in large amplitude shear mode (at strains

in excess of 100%) The SAOS measurement is a non-destructive technique

to get an insight of the material characteristics of any food material

Strain Sweep Test

The domain of linear viscoelasticity is established by the oscillatory strain sweep experiment Here, the strain amplitude of the oscillatory shear fl ow

at a fi xed frequency is continuously increased until the dynamic properties (storage- and loss-moduli) change signifi cantly with strain Below this strain level, viscoelastic response is linear Strain sweeps at different frequency are performed for all the samples at selected temperatures to determine the linear viscoelasticity zone

Frequency Sweep Test

After obtaining the value of strain, for which the material exhibited linear behaviour, the frequency sweep tests are performed at specifi ed strain level over a frequency range The oscillatory rheological parameters obtained are:

storage modulus (G’), loss modulus (G”), complex modulus (G*), complex

dynamic viscosity (η*), and loss angle (tan δ).

Shear Sweep Test

The shear sweep test is carried out over a selected range of the shear rate

values The apparent viscosity (η) against shear rate (γ) data can be used

with complex viscosity (η*) against frequency (ω) data to test the validity

of the Cox-Merz rule

1.6.2 Analysis of Dynamic Rheological Data

The power law describes the rheological behaviour of an incipient gel

within the linear viscoelastic region because the frequency dispersions of

the dynamic mechanical spectra (G’ and G”) are more or less straight lines

with different slopes Therefore, each set of data can be fi tted by power law

where a is the low-frequency storage modulus (Pa); b is the power law index

for storage modulus (dimensionless); c is the low-frequency loss modulus

(Pa); and d is the power law index for the loss modulus (dimensionless).

Cox-Merz Rule

Several empirical relations have been proposed to relate the viscometric

functions to linear viscoelastic properties The Cox–Merz rule is one such

The Cox–Merz rule is a simple relationship that predicts whether the

complex viscosity ǀη*(ω)ǀ and steady shear viscosity η(γ

) are equivalent when

the angular frequency (ω) is equal to the steady shear rate (γ

)

Although originally developed for synthetic polymers (Steffe 1996),

the Cox-Merz rule and its modifi ed forms have been applied to many

liquid and semisolid foods (Bistany and Kokini 1983; Yu and Gunasekaran

2001) Deviation from the Cox-Merz relation has been attributed to various

structural aspects and to particle-particle interactions, especially in highly

concentrated systems (Yu and Gunasekaran 2001; Gleissle and Hochstein

2003) Compared to synthetic polymers, rheological behaviour of food materials may deviate from the Cox-Merz relation to a large extent However, in many cases, it has been found that the foods follow the same

general behaviour when a shift factor, A, is introduced (Bistany and Kokini

1983; Yu and Gunasekaran 2001; Gleissle and Hochstein 2003)

where K / and α are constants In most cases both η* versus ω and η versus

γ . may be approximated by a power law Thus, when α = 1, this relation

reduces to the modifi ed Cox-Merz rule

Weak Gel Model

Gabriele et al (2001) were the fi rst to conceptualize food as a critical weak gel and proposed a power law relaxation modulus to describe the rheological behaviour of dough, jam, and yoghurt This descriptive framework has also been used recently to explore other types of deformation including creep relaxation, and uniaxial and biaxial extension (Gabriele et al 2004; Ng and Mckinley 2008) The so called ‘weak gel model’ (Gabriele et al 2001) is extremely attractive because of its relative functional simplicity

This model provides a direct link between the microstructure of the material and its rheological properties The most important parameter

introduced is the ‘coordination number’, z, which is the number of fl ow

units interacting with each other to give the observed fl ow response Above the Newtonian region, there exists a regime characterized by the following

fl ow equation:

where, A is a constant that may be interpreted as the ‘interaction strength’

between the flow rheological units Thus, material functions of food system in the linear viscoelastic regime may be well described by only two

parameters (A and z).

where, k 1 (Pa.s) and k 2 (Pa) are constants; (1/ k 1) represents the initial decay

rate; and (1/k 2) is the asymptotic strain

where J o is the instantaneous compliance re sulting from the instantaneous

stress applied; J 1 is the time-dependent retarded elastic compliance (Kelvin

component); λ ret is the retardation time of the Kelvin component; and µ o

is the asymptotic viscosity of the material (viscous fl ow of the bond-free

constituents) The instantaneous compliance (J o) is the compliance at time zero and is determined by extrapolation of the compliance to zero time

Kelvin Model

The Kelvin model (Steffe 1996) is a six-parameter mechanistic model, being

an extension of the Burgers model: